Since M2O3 type sesquioxide sintered bodies belong to the cubic system of space group, like the garnet structure represented by YAG sintered bodies or the like, they are free of optical anisotropy. Theoretically, therefore, the M2O3 type sesquioxide sintered bodies with high linear transmission properties are expected to be obtainable. In addition, unlike the sintered bodies of garnet structure materials, the M2O3 type sesquioxide sintered bodies do not need any B site element which will not directly contribute to the above-mentioned optical uses. Therefore, it is expected that the addition concentration of a specified element for exhibiting an optical function can be enhanced. Furthermore, it is expected that M2O3 type sesquioxide sintered bodies in which at least two elements selected from the group consisting of Y and the lanthanide elements are dissolved in arbitrary atomic ratio on a solid basis can be obtained in a stable manner. This leads to an expectation that the degree of freedom in designing a transparent sintered body exhibiting an optical function can be enhanced remarkably. Accordingly, developments of the M2O3 type sesquioxide sintered bodies have been made vigorously in recent years.
For instance, JP-A H05-330912 (Patent Document 1) discloses polycrystalline transparent Y2O3 ceramics for laser use wherein the porosity of the sintered body is up to 1%, the average particle diameter is in the range from 5 to 3000 μm, and at least one lanthanide element is contained. It is described in the document that polycrystalline transparent Y2O3 ceramics for laser use which permit enhancement of the concentration of a luminous element (particularly, Nd) and enhancement of the uniformity of the inside of the sintered body can be obtained.
Besides, JP-A H06-211573 (Patent Document 2) discloses a transparent Y2O3 sintered body. For obtaining the sintered body, a Y2O3 powder is used which has a purity of at least 99.8 wt %, has a primary particle average diameter in the range from 0.01 to 1 μm, and which shows a theoretical sintered body density of at least 94% when molded at a hydrostatic pressure of 500 kg/cm2 and sintered at normal temperature at 1600° C. for three hours. The Y2O3 powder is kneaded and dried, is then molded into a predetermined shape, and the molded body is sintered in oxygen, hydrogen, or a vacuum of up to 10−4 Torr in a temperature range of 1800 to 2300° C. for at least three hours, to obtain the transparent Y2O3 sintered body. It is described in the document that by this method, a transparent Y2O3 sintered body can be obtained while using a material system not including ThO2, which is radioactive, or while using only a pure Y2O3 powder not containing LiF, BeO or the like.
Further, JP-A H11-157933 (Patent Document 3) discloses a transparent ceramic including a dense polycrystalline sintered body which has a composition formula R2O3 (R is at least one member selected from among yttrium and lanthanoids) and an average particle diameter of up to 50 μm (exclusive of 0 μm). It is described in the document that the transparent ceramic promises provision of arc tubes and high-voltage discharge lamps with long life.
Furthermore, JP 4033451 (Patent Document 4) discloses a light-transmitting rare earth oxide sintered body having an average particle diameter of 2 to 20 μm. The sintered body is represented by the general formula R2O3 (R is at least one member of the group consisting of Y, Dy, Ho, Er, Tm, Yb, and Lu), has a linear transmittance of at least 80% at sintered body thickness of 1 mm in the wavelength range of 500 nm to 6 μm excluding the unique absorption wavelength thereof, has an Al content of 5 to 100 ppm by weight on metal basis, and has a Si content of up to 10 ppm by weight on metal basis. It is described in the document that a transparent R2O3 sintered body wherein a sintering assistant would not be segregated as a different phase can be obtained.
In addition to the above, JP-A 2008-143726 (Patent Document 5) discloses a polycrystalline transparent Y2O3 ceramic for electron-beam fluorescence including a polycrystalline sintered body containing Y2O3 as a main ingredient. The polycrystalline sintered body has a porosity of up to 0.1%, has an average crystal particle diameter of 5 to 300 μm, and contains a lanthanide element. It is described in the document that a transparent Y2O3 ceramic for electron-beam fluorescence enabling uniform distribution of a fluorescent element and addition of the element in a high concentration can be obtained.
Besides, JP-A 2009-23872 (Patent Document 6) discloses a method of manufacturing a transparent M:Y2O3 sintered body (M is at least one element selected from the group consisting of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Ti, V, Cr, Mn, Fe, Co, and Ni). The method includes: a step of preparing a starting solution including Y(NO3)3 and a nitrate of M (M is the selected element); a step of adding an aqueous NH3 solution to the starting solution; a step of further adding (NH4)2SO4 to the reaction solution obtained by the above-mentioned step; a step of calcining a powder obtained from the reaction solution; a step of molding the calcined powder; and a step of sintering the molded powder. It is described in the document that it is possible to provide, by the method, an M:Y2O3 sintered body applicable to laser medium, at a comparatively low temperature, speedily, and without using any special apparatus.
Furthermore, JP-A 2007-334357 (Patent Document 7) discloses a refractive, transmissive or diffractive optical element having a three-dimensional structure which contains a ceramic comprised of a combination of crystals, with the single microcrystal thereof resembling that of Y2O3, is transparent to visible rays and/or infrared rays, and contains one or more oxides of X2O, type. It is described in the document that a lens element free of absorption in the visible band can be obtained.
Recently, JP-A 2011-121837 (Patent Document 8) has disclosed a light-transmitting terbium oxide sintered body for magnetooptical element which is a cubic polycrystalline sintered body containing Tb2O3 as a main ingredient. The sintered body has a porosity of up to 0.2%, has linear transmittances at wavelengths of 1.06 μm and 532 nm per 3 mm length of at least 70%, and contains at least 2×1022 Tb3+ ions per 1 cm2. It is described in the document that it is possible to provide a sintered body with excellent light transmission properties, which is free of absorption in a wavelength range of 400 to 1100 nm, except for the absorption in a narrow width near 500 nm attributable to the trivalent terbium ions.
In this way, developments of M2O3 type sesquioxide sintered bodies have been made vigorously in recent years.